Method for the Manufacture of a Polyhydroxy-Carboxylic Acid

ABSTRACT

Disclosed is a method to prepare a polylactic acid comprising the steps of: performing a ring opening polymerization using a catalyst and either a catalyst killer compound or an endcapping additive to obtain a raw polylactic acid of MW greater than 10,000 g/mol, purifying the raw polylactic acid by removing and separating low boiling compounds comprising lactide and impurities from the raw polylactic acid by devolatization of the low boiling compounds as a gas phase stream, purifying the lactide from the devolatization and removing the impurities from the gas phase stream of evaporated low boiling compounds by means of condensing the evaporated gas phase stream to give a condensed stream and a subsequent melt crystallization of the condensed stream, wherein the lactide is purified and the removed impurities include a catalyst residue and a compound containing at least one hydroxyl group such that the purified lactide is polymerized by feeding it back into the ring opening polymerization. The invention further relates to an apparatus for carrying out the method comprising: a polymerization reactor for performing a ring opening polymerization to obtain a raw polylactic acid, a devolatization apparatus for removing and separating low boiling compounds comprising lactide and impurities from a raw polylactic acid, and a crystallization apparatus for purifying a lactide and removing impurities from a condensed stream, wherein a condenser for condensing a gas phase stream to give a condensed stream is arranged between the devolatization apparatus and the crystallization apparatus.

This invention relates to a method for the manufacture of apolyhydroxy-carboxylic acid, in particular a polylactic acid in whichthe yield is increased with respect to the final product by recycling ofthe lactide from a side stream stemming from the purification of rawpolylactic acid and the recycling of the lactide obtained resulting fromthis purification. Furthermore the invention relates to an apparatus forcarrying out the method to produce a polylactic acid. The invention alsorelates to a method for a melt layer crystallization of a vaporousbiodegradable, intermolecular cyclic diester of analpha-hydroxy-carboxylic acid.

Polylactic acid, which will be also referred to as PLA in the subsequenttext, is a biodegradable polymer, which is synthesized from lactic acid.A particular advantage of such polymers is their biocompatibility. Bythe term biocompatibility, it is meant that they only have a verylimited negative influence on any living creatures in the environment. Afurther advantage is that polylactide polymers are derived from anentire renewable raw material, such as starch and other saccharidescoming from e.g. sugar cane, sugar bets and the like.

Polylactide polymers have been increasingly commercialized already sincemid of 20^(th) century. However, mainly due to restricted monomeravailability and high manufacturing costs, their original use was mainlyin the medical sector, such as chirurgical implants or surgical sutures,e.g. nails, screws, sewing material or reinforcing material for bonefractures. An interesting property of the PLA is the decomposition ofthe polylactide polymers in the body saving a second surgical treatmentfor removing any implant. Furthermore, PLA can be used in sustainedrelease capsules for the controlled dispensing of drugs.

In the recent decades, due to strongly increasing crude oil prices andenvironmental awareness along with improvements of production methods,making the polylactide polymers became more interesting for packaging inparticular of foodstuff, both as stiff packaging as well as flexiblefoils, such as mono-axially or bi-axially stretched films. Otherapplication are fibers e.g. for textiles used in garments, furnitureupholstery or carpets. Furthermore, extrusion products like one-waycutlery, or containers, office supplies or hygienic articles. Thepolylactide polymers can also be combined with other materials to formcomposite materials.

Currently, two production methods are known for manufacturing PLA. Thefirst of these production methods includes the direct polycondensationof lactic acid to polylactic acid, as described in JP733861 orJP5996123. A solvent is used in addition to the lactic acid forperforming the polycondensation reaction. Furthermore water has to bedischarged continuously during the entire polycondensation process inorder to allow for the formation of polylactide polymers of highmolecular weight. For all these reasons, this method has not beencommercialized.

The method that has been established for commercial PLA manufacture usesthe intermediate product lactide to initiate a subsequent ring openingpolymerization leading from the lactide to polylactic acid. A number ofvariants to this methods have been disclosed e.g. in U.S. Pat. No.5,142,023, U.S. Pat. No. 4,057,537, U.S. Pat. No.5,521,278, EP261572JP564688B, JP2822906, EP0324245, WO2009121830. The methods described inthese documents have the following main steps in common: In a firststep, the raw material is processed, e.g. starch or other saccharidesextracted e.g. from sugar cane or beets, corn, wheat in a second step afermentation using suitable bacteria to obtain lactic acid is performed,in a third step the solvent, typically water, is removed, from themixture to be able to work without solvents in the subsequent steps. Ina fourth step the lactic acid is catalytically dimerized to form a rawlactide. Typically an optional intermediate step is performed, whichincludes a pre-polymerization of the lactic acid to a low molecularweight polylactic acid and a subsequent de-polymerization to form a rawlactide. A fifth step includes the purification of the lactide to removeforeign substances, which may influence the polymerization in a negativemanner and contribute to the coloring as well as the odor of the finalproduct. The separation can either be performed by distillation or bycrystallization. In a sixth step, a ring opening polymerization forobtaining raw polylactic acid of high molecular weight is obtained. Themolar mass is approximately 20000 to 500000 g/mol according to U.S. Pat.No. 6, 187,901. Optionally copolymerization compounds can be addedduring the ring opening polymerization. In a seventh step, the rawpolylactic acid is purified to obtain a purified polylactic acid. Inthis stage, low boiling compounds are removed, which would decrease thepolymer stability and in a negative way influence the parameters ofsubsequent plastics manufacture, like viscosity or rheologicalproperties of the molten plymer which would contribute to the coloringand unwanted odors of the final product. According to U.S. Pat. No.5,880,254,the raw polylactic acid may be solidified to form a granulatewhich is contacted with a tempered inert gas flow e.g. in a fluid bed.The lowest boiling compounds of the raw polylactic acid are carried awayby the inert gas. Yet another method is described in U.S. Pat. No.6,187,901. According to this method, the liquid raw polylactic acid issprayed by a plurality of nozzles so as to form a plurality of liquidthreads. The inert gas passes around the liquid threads and the lactideevaporates into the hot inert gas flow. The flow of low boilingcompounds typically contains up to 5% weight of dilactide.

The lactic acid has two enantiomers, L-lactic acid and D-lactic acid.Chemically synthesized lactic acid contains the L-lactide and D-Lactidein the racemic mixture of 50% of each of the enantiomers. However, thefermentation process is made more selective by using appropriate microbcultures to selectively obtain L- or D-lactic acid.

The lactide molecules which are produced by the dimerization of thelactic acid appear in three different forms: L- L lactide, which is alsocalled L-lactide, D-D lactide, which is also called D-lactide and L, Dlactide or D, L lactide, which is also called meso-lactide. L andD-lactides are optically active, whereas meso-lactide is not. Thepurification steps for purifying raw lactide typically include aseparation of a stream rich in L-lactide and a stream rich in D-lactideand a further stream rich in meso-lactides, each of which can bepurified separately. By blending at least two of the three lactideforms, the mechanical properties and the melting point of the polymersformed by the polylactic acid can be influenced. For example, byadmixing appropriate amounts of one enantiomer to the other, thecrystallization rate of the polymer is decreased, that in turn allowsfoaming of the manufactured plastic mass without being obstructed by atoo rapid solidification.

Attempts have been made to increase the yield of the polylactic acidprocess and to reduce manufacturing costs for polylactic acid.

U.S. Pat. No. 5,142,023 teaches that the gaseous stream of the lowboiling compounds of the purification step of the raw lactide are fed atleast partially back into the lactide reactor. A heavy residue forms inthe lactide reactor, which can be partially bypassed back into thereactor itself or fed back into the separation device for separating thesolvent from the lactic acid after fermentation.

U.S. Pat. No. 7,488,783 teaches that raw lactide is crystallized to forma purified lactide. A second crystallization step is performed onto theresidue of the first crystallization step to separate the lactidetherefrom. This lactide is fed back to the first crystallization step orto one of the previous process steps according to the method.

U.S. Pat. No. 5,521,278 teaches that the raw lactide is crystallized.The residue flow is evaporated, condensed selectively and recycled backto one of the previous process steps according to the method.

JP2822906 discloses the solidification of a gaseous raw lactide streamto pure lactide. The residue, which is not solidified is recycled backinto the lactide reactor.

JP10101777 discloses that the gaseous raw lactide stream is solidifiedpartially by a cooling inert gas stream to form pure lactide The residueis fed back into the lactide reactor. This raw lactide stream stems froma direct polycondensation reaction. This raw lactide stream is gaseous.By cooling the raw lactide stream generated by said polycondensationreaction to a temperature in which the lactide crystallizes in acrystallization flowing-back equipment with a self cleaning function.This crystallization flowing-back equipment has a rotary driving meansfor rotating two screws arranged in a cylinder, whereby the rotatingscrews are disposed with intermeshing gears. The cylinder is cooled by acooling medium circulating in a cooling jacket arranged in the cylinderwall to the temperature in which a part of the low molecular weightcompound of lactide and lactic acid crystallizes and is conveyed towardsthe vent-port by the two screws and flows back from this vent-port tothe batch process polycondensation reactor. The crystallization isperformed by using a solvent. Such a solvent, e.g. water is used tolower the viscosity of the melt, which is believed to improve the masstransfer. Therefore the low melting compounds separate more readily fromthe high melting compounds, which form a crystal fraction on thecrystallization surface of the crystallization apparatus. Thus acontamination of the crystals is believed to be reduced if the viscosityof the melt is reduced. The object of the invention as disclosed inJP10101777 is to remove the solvent.

Any of the described methods concern a recycle of partial stream fromthe raw lactide purification. Any of these methods serves to increasethe yield of the method, however do not disclose if the lactide can berecycled which is still present in the raw polylactic acid at apercentage of up to 5%.

Document U.S. Pat. No. 6,187,901 relates to a method for the removal oflactide from polylactide and the recovery of lactide from alactide-containing gas. The raw polylactic acid is sprayed into a spacecontaining a hot inert gas by means of spray nozzles. Thereby thinthreads are formed. These threads fall under gravity and under laminarflow conditions. Thereby the polymer melt flows more rapidly into theinner parts of the thread than in the surface part. Thereby the polymermelt flowing in the inner part of a sufficiently thin thread forms a newmaterial transfer surface for lactide evaporation during its downwardpath. The lactide evaporates partially and is collected in the inertgas, from which it crystallizes in a crystallization chamber by rapidcooling. The crystals obtained are separated in a cyclone or filterdevice and recycled into the polymerization reactor. The amount oflactides in the polylactic acid can be reduced by this process step upto 1%. However, the lactide recycling requires an inert gas flow, whichhas to be cleaned before discharge as a waste stream.

Document U.S. Pat. No. 5,880,254 discloses a method for producingpolylactic acid. The raw polylactic acid is crystallized in the form ofa granulate. The granulate is subjected to a hot inert gas flow passingthrough the granulate forming a fluid bed. The lactide contained in thegranulate is evaporated and carried away with the inert gas flow and fedback into the polymerization reactor. The purified polylactic acidcontains still about 1% of dilactide.

Each of the methods of U.S. Pat. No. 6,187,901 or U.S. Pat. No.5,880,254 require an inert gas which has to be treated for recyclingthat in turn requires additional equipment having the consequence ofincreased costs for the purification of the polylactic acid.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved method forpreparing a polylactic acid not having the disadvantages of the earlierdiscussed methods, and a further object is to reduce the equipmentneeded for treatment of the inert gas and to increase the yield comparedto the methods according to U.S. Pat. No. 6,187,901 or U.S. Pat. No.5,880,254.

According to the invention, the first object is achieved by a methodmethod to prepare a polylactic acid comprising the steps of performing aring opening polymerization using a catalyst and either a catalystkiller compound or an endcapping additive to obtain a raw polylacticacid of MW greater than 10,000 g/mol, purifying the raw polylactic acidby removing and separating low boiling compounds comprising lactide andimpurities from the raw polylactic acid by devolatization of the lowboiling compounds as a gas phase stream, purifying the lactide from thedevolatization and removing the impurities from the gas phase stream ofevaporated low boiling compounds by means of condensing the evaporatedgas phase stream to give a condensed stream and a subsequent meltcrystallization of the condensed stream, wherein the lactide is purifiedand the removed impurities include a catalyst residue and a compoundcontaining at least one hydroxyl group such that the purified lactide ispolymerized by feeding it back into the ring opening polymerization.

The further object is achieved by an apparatus for carrying out themethod comprising a polymerization reactor for performing a ring openingpolymerization to obtain a raw polylactic acid, a devolatizationapparatus for removing and separating low boiling compounds comprisinglactide and impurities from a raw polylactic acid, and a crystallizationapparatus for purifying a lactide and removing impurities from acondensed stream, wherein a condenser for condensing a gas phase streamto give a condensed stream is arranged between the devolatizationapparatus and the crystallization apparatus.

In a preferred embodiment of the method, the melt crystallization isperformed by means of a layer or a suspension crystallization. Inanother preferred embodiment of the method, the evaporated gas phasestream from the devolatization contains at least 30% of lactide,preferably at least 60%, most preferred at least 90%. In yet anotherpreferred embodiment, a crystal resulting from the melt crystallizationof the condensed stream is crystallized in a further crystallizationstage. In still yet another preferred embodiment, the layercrystallization comprises a sweating step followed by a melting step ofa solidified fraction present in a crystalline form on a crystallizationsurface. In still yet another preferred embodiment, the removedimpurities include either an organometallic compound or a carboxylicacid. In still yet another preferred embodiment, an apparatus is usedfor the melt crystallization which does not have an inert gas stream. Instill yet another preferred embodiment, at least a portion of a purgestream from the crystallization is recycled to a raw lactidepurification step, a pre-polymerization and dimerization step, or asolvent removal step in the production of a purified lactide. In stillyet another preferred embodiment, a mother liquor from thecrystallization and/or a liquid from the sweating step is collected andrecrystallized in order to recover the lactide.

In a preferred embodiment of the apparatus of the invention, thecrystallization apparatus is a layer crystallization apparatus or asuspension crystallization apparatus. In another preferred embodiment,the layer crystallization apparatus is a static or a falling filmcrystallization apparatus. In yet another preferred embodiment, thesuspension crystallization apparatus contains a wash column.

DETAILED DESCRIPTION OF THE INVENTION

An object of the invention is a method comprising purification ofpolymerizable monomers or oligomers like lactide by crystallization, inwhich in a first step, a ring opening polymerization for obtaining rawpolylactic acid of high molecular weight of greater than 10 000 g/mol isperformed;

in a second step, the raw polylactic acid is purified to obtain apurified polylactic acid whereby during the second step, low boilingcompounds are removed and the separation of the low boiling compoundsfrom the raw polylactic acid is achieved by devolatization and in athird step, the lactide is recycled and impurities are removed from theevaporated gas phase stream of the second step by means ofcrystallization or solidification from the gas phase. During the thirdstep, the impurities are removed such that the purified lactide can beadded again to the ring opening polymerization of the second step. Suchimpurities can comprise coloring and odor generating compounds or anyadditive byproduct, such as water, catalyst residues, e.g.organometallic compounds, reaction byproducts, compounds containing atleast one hydroxyl-group (-OH), acidic compounds, such as carboxylicacids, catalyst killer compounds or endcapping additives.

Advantageously the molecular weight of the raw polylactic acid is atleast 10 000 g/mol, preferably at least 15 000 g/mol, particularlypreferred at least 20 000 g/mol. Optionally other polymerizable monomersor oligomers can be included, such as at least one of the group of aglycolactide copolymer, a polyglycolic acid or polyglycolide acid (PGA),a block copolymer a styrene-butadiene-methacrylate (SBM) copolymer ofpolystyrene, 1, 4-polybutadiene, a syndiotactic poly methyl methacrylate(PMMA), a triblock copolymer with a center block of poly butyl acrylatesurrounded by two blocks of poly methyl methacrylate, poly methylmethacrylate (PMMA), polyether ether ketone (PEEK), polyethylene oxide(PEO), polyethylene glycol (PEG), polycaprolactam, polycaprolactone,polyhydroxybutyrate. Typical comonomers for lactic acid or lactidecopolymerization are glycolic acid or glycolide (GA), ethylene glycol(EG), ethylene oxide (EO), propylene oxide (PO), (R)-β-butyrolactone(BL), 5-valerolactone (VL), c-caprolactone, 1,5-doxepan-2-one (DXO),trimethylene carbonate (TMC), N-isopropylacrylamide (NIPAAm). The rawpolylactic acid may also contain further impurities.

At the end of polymerization the temperature dependent equilibriumbetween the monomer and the polymer is reached, whereby the rawpolylactic acid contains about 5 weight % of non-reacted lactide. Themonomer content has to be reduced to less than 0.5 weight % in order toobtain the required mechanical, chemical, rheologic and thermalproperties of the polymer for further processing thereof.

The evaporated gas phase stream leaving the devolatization can becondensed, whereby a condensed stream is obtained. The evaporated gasphase stream contains at least 30% of lactide by weight. The impuritiesshould be present only in small amounts, thus water should be at most 10ppm, preferably 5 ppm, particularly preferred less than 0.5 ppm. Anylactic acid in the evaporated gas phase stream should be below 100mmol/kg, preferably less than 50 mmol/kg, particularly preferred lessthan 10 mmol/kg. The condensed stream is crystallized from its liquidstate and the crystallization is advantageously performed withoutsolvent. This has the particular advantage, that further steps to removeany solvents are not required. Advantageously, the crystallization stepis performed in one of a melt layer crystallization apparatus or adesublimation apparatus, such as at least one of a falling filmcrystallizing apparatus or a static crystallization apparatus, or asuspension crystallization, which is performed in at least onesuspension crystallization apparatus. If a suspension crystallizationapparatus is used, the condensed stream is cooled so as to form lactidecrystals floating freely in the liquid phase of the suspensioncrystallization apparatus, thereby forming a partially crystallizedliquid stream, which is subsequently fed into a wash apparatus.

As an alternative, the evaporated gas phase stream can be desublimized,thus cooled from the gas phase directly to the solid phase in adesublimation step.

The crystal fraction obtained by the crystallization according any ofthe alternatives outlined above contains the purified lactide.Advantageously the devolatization operates under a lactide partialpressure of less than 20 mbar, preferably less than 10 mbar,particularly preferred less than 5 mbar. The solidified fractioncontaining purified lactide may be melted in a subsequent heating stepto be fed back into the ring opening polymerization. A sweating step canbe performed before the heating step for the solidified fraction presentin crystalline form on the crystallization surfaces. The mother liquorcan remain between the crystal and thereby form inclusions containingimpurities. During the sweating step, these impurities are removed.

The evaporated gas phase stream from the devolatization contains atleast 30% of lactide, advantageously at least 60% of lactide, mostpreferred at least 90% of lactide. For increasing the yield of thelactide from the evaporated gas phase stream, the mother liquor and /orliquid from sweating stage can be fed into a recrystallization stage.

According to a preferred embodiment of the invention, thecrystallization apparatus is connected directly to the devolatizationapparatus by means of a gas line or optionally a heat exchanger arrangedbetween the devolatization and the crystallization. The heat exchangeris configured in particular as a gas cooler. Such a heat exchanger isparticularly advantageous to reduce the desublimation surface of thecrystallization apparatus since part of the sensible heat can already beremoved from the vapor stream before entering the crystallizationapparatus.

The direct connection between the crystallization apparatus and thedevolatization apparatus has the effect that both devices operatesubstantially under the same vacuum conditions. That means that nothrottling means or vacuum pumps are arranged between thecrystallization apparatus and the devolatization apparatus.

It has been found by the inventors, that the viscosity of the condensedlactide fraction in a melt crystallization step surprisingly allows to asufficient mass transfer and in turn a sufficient purification of thecrystal fraction. Melt crystallization is to be understood a as acrystallization, which is solvent-free. The viscosity of the melt can beup to 100 mPas, whereby the viscosity is preferably lower than 10 mPas,particularly preferred lower than 5 mPas.

According to a preferred embodiment, the method comprises a first step,in which a raw material is processed for the extraction of fermentablepolysaccharides. The raw material may stem from corn, sugar plants,cane, potatoes or other sources o fermentable polysaccharides. In asecond step a fermentation using suitable bacteria to obtain a rawlactic acid is performed. In a third step the solvent is removed fromthe mixture. According to a preferred method, the solvent may be removedby evaporation. The solvent can in particular be water. In a fourth stepthe lactic acid is catalytically dimerized to form a raw lactide. Anoptional intermediate step can be performed, which includes apre-polymerization of the lactic acid to a low molecular weightpolylactic acid and subsequent depolymerization to form a raw lactide.The lactic acid, which has not been reacted to raw lactide can bedrained and be recycled to the apparatus for performing the third step.The heavy residues from the lactide reactor can be recycled to thereactor of any of the second or third steps. A portion of the heavyresidues can also be added to the subsequent sixth step, which includesthe polymerization of the purified lactide to polylactic acid or can berecycled to the apparatus for performing the third step.

In a fifth step the purification of the lactide is performed to removeforeign substances, which may influence the polymerization in a negativemanner and contribute to the coloring as well as the odor of the finalproduct. The separation can either be performed by distillation or by acrystallization process. The unwanted compounds, such as non-reactedlactic acid, other carboxylic acids are contained in the vapor phase,when evaporation is used. These unwanted compounds are present in thenon-crystallized residue. The stream of unwanted compounds may berecycled to any of the apparatus of the third or fourth steps.

In a sixth step, a ring opening polymerization for obtaining rawpolylactic acid of high molecular weight is obtained. Duringpolymerization the temperature dependent equilibrium between the monomerand the polymer is reached. The raw polylactic acid contains about 4 to6 weight % of non-reacted lactide. The monomer content has to be reducedto less than 0.5% in order to obtain the required mechanical propertiesof the polymer for further processing thereof. Therefore the rawpolylactic acid has to be purified.

In a seventh step, the raw polylactic acid is purified to obtain apurified polylactic acid. In this stage, low boiling compounds areremoved, which habitually contribute to the coloring and unwanted odorsof the final product or may contain additives, which would have anundesired effect on the ring opening polymerization process if recycled.The separation of the low boiling compounds from the raw polylactic acidis achieved by devolatization for example by flash evaporation undervacuum conditions. The evaporated stream contains at least 30% oflactide, which has not been reacted to polylactic acid during the ringopening polymerization according to the sixth step. Furthermore theevaporated gas phase stream may contain other low boiling compounds,which contribute to the coloring or smell of the final product, both ofwhich are mostly unwanted properties, reaction by-products or additiveshaving any undesired effect on the ring opening polymerization ifrecycled. The purification according to the seventh step may beperformed in one or more subsequent devolatization stages. The mainportion of the lactide contained in the raw polylactic acid stream isretained in the first devolatization stage, which amounts to a majorportion of the total of 5%.

In an eighth step, the lactide is purified and recycled from theevaporated gas phase stream of the seventh step by means ofcrystallization, which can comprise a desublimation, thus asolidification from the gas phase. During this step, the coloring andodor generating compounds or undesired additives are removed such thatthe purified lactide can be added again to the ring openingpolymerization of the sixth step, thereby preventing any accumulation ofsuch coloring and odor generating compounds or acting in detrimental wayto the process in the sixth process step.

The lactide content of the purified PLA leaving the devolatization as aproduct stream is less than 1%. Preferably, the lactide content of thepurified PLA is less than 0.5 weight %.

The lactide content of the evaporated gas phase stream at least 30%weight, preferably at least 60%, most preferred at least 90%.

According to a variant of the method according to the invention, theevaporated stream leaving the devolatization is condensed andcrystallized from its liquid state. Such a crystallization can beperformed without solvent as a layer crystallization in a falling filmcrystallizing apparatus or a static crystallization apparatus.Alternatively, the crystallization can be performed in a suspensioncrystallization apparatus, in which the condensed mixture is cooled sofar as to form lactide crystals floating freely in the liquid therebyforming a partially crystallized liquid stream. This partiallycrystallized liquid stream is fed into a wash apparatus, in which theseparation of the solid from the liquid residue is performed.

The crystal fraction obtained by any of the crystallization apparatusesmentioned above contains the purified lactide and is melted in the lastcrystallization stage to be fed back into the ring openingpolymerization according to the sixth step. The non-crystallized motherliquor has to be drawn off from the process as a waste stream or it canbe at least partly recycled to any of the above-mentioned upstreamprocess steps e.g. 3, 4, 5 as shown in FIG. 2.

According to a variant of the method according to the invention, thecrystallization apparatus, in which the lactide crystals are formed isto be connected directly to the devolatization apparatus. Thedevolatization operates under a lactide partial pressure of less than 20mbar, preferably less than 10 mbar, particularly preferred less than 5mbar. The lactide from the evaporated gas phase stream is solidifiedonto the cooled crystallization surfaces provided by the crystallizationequipment forming crystallization layers. The solidified fractioncontaining purified lactide is melted in a subsequent heating step to befed back into the ring opening polymerization according to the sixthstep. The liquid fraction, which had not been deposited as crystals onthe crystallization surfaces, has to be drawn off from the process as awaste stream.

The heating step to melt the crystals on the crystallization surfacescan be preceded by a sweating step. During the sweating step a partialmelting of the crystals is performed. Any remainders of unwantedcompounds present between the crystals of polycrystalline layers or onthe surfaces thereof can be separated and removed from the lactidecrystals. Under a polycrystalline layer, a layer is understood whichcontains a plurality of crystals. Between the crystals of such apolycrystalline layer, impurities can accumulate. These impurities maybe disposed of by the sweating step. The liquid fraction generatedduring the sweating step has to be drawn off from the process as a wastestream.

In a layer crystallization, the polycrystalline layers are formed onheat exchanging surfaces provided by the crystallization apparatus.According to a preferred embodiment the heat exchanging surfaces areplates or tubes through which a cooling medium circulates. Acrystallization apparatus having plates as heat exchanging surfaces isalso known as a static crystallization apparatus. A crystallizationapparatus having tubes as heat exchanging surfaces is also known as afalling film crystallization apparatus.

In order to increase the purity of the lactides generated from theevaporated gas phase stream of the devolatization, the layercrystallization can be performed in a plurality of stages. The moltencrystals resulting from the crystallization of the liquefied evaporatedgas phase stream can by crystallized in a further crystallization stage,whereby the purity of the crystallization fraction resulting from thissecond crystallization step is crystallized anew, whereby the purity ofthe crystals of the second stage is increased. The liquid residue fromthe second crystallization stage can be fed back together with anyliquid fraction from a sweating step to the feed for the firstcrystallization stage.

It is possible to foresee more than two crystallization stages, wherebythe liquid residue from the last crystallization stage can be fed backtogether with any liquid fraction from a sweating step to the feed ofany one of the preceding crystallization stages. The optimum number ofcrystallization stages depends on the required purity of the lactide.

Furthermore, the crystals generated by solidification from the gas phasecan be molten and then be recrystallized for increasing the purity ofthe lactide.

According to a further variant for increasing the yield of the lactidefrom the gas evaporation stream, the mother liquor and or liquid fromsweating stage can be collected and be recrystallized in order torecover the lactide contained still in the two fractions.

The mother liquor from the first crystallization step, thus theliquefied evaporated gas stream is crystallized to obtain the lactide ascrystallized fraction, so that the content of the lactide in the motherliquor and/or liquid from the sweating stage of this recrystallizationstage is lower than in the corresponding fraction of the crystallizationof the liquefied evaporated gas stream. The crystallizate of such arecrystallization stage can also be submitted to a sweating step andsubsequently melted to be added to the liquefied devolatizationfraction. It is possible to employ further recrystallization stages,whereby the content of the lactide in the liquid residue and/ or theliquid from the sweating step of a subsequent recrystallization stage isreduced compared to each previous recrystallization stage. Thereby themother liquor and/or the liquid from the sweating step of a subsequentrecrystallization stage are fed into a previous recrystallization stageand the molten crystallizate is fed into a subsequent recrystallizationstage. The number of recrystallization stages is determined by a costoptimization over the entire process.

The layer crystallization in the embodiment of a melt crystallization ora solidification from the gas phase, that is a desublimation, are batchprocesses. Advantageously these steps are performed in one or morecrystallization apparatuses, such as a melt crystallization apparatus ora desublimation apparatus. The working sequence of these apparatuses isadvantageously staged so as to perform a crystallization ordesublimation in one of the apparatuses while performing a sweating ormelting in any of the other apparatuses. In such a way a continuousdischarge of the evaporated gas phase stream for crystallization isguaranteed without the need of intermediate buffering.

A notable advantage of the recycle of the lactide from an evaporated gasphase stream from the devolatization apparatus is the use of lesscomplicated equipment and a simpler process as compared to the prior artsuch us the processes disclosed in U.S. Pat. No. 6,187,901 or U.S. Pat.No. 5,880,254. The crystallization apparatus is of a simple mechanicalconstruction. Furthermore no inert gas streams are required, thus anytreatment steps for such an additional inert gas stream are notnecessary, which results in substantial cost advantages in favor of thelactide regeneration process according to the invention.

A further object of the invention is to improve the purification of avaporous biodegradable, intermolecular cyclic diester of analpha-hydroxy-carboxylic acid and to keep the waste as small as possibleand to reduce the equipment to perform the purification.

This object is achieved by a method for the melt layer crystallizationof a vaporous biodegradable, intermolecular cyclic diester of analpha-hydroxy-carboxylic acid of the formula I

wherein R is chosen from hydrogen or one of a linear or branchedaliphatic radical having one to six carbon atoms of a melt streamcontaining the diester of the formula I. In particular, the temperatureof the melt stream when entering a melt layer crystallization apparatusfor performing the melt layer crystallization is adjusted to between 0°C. and 130° C., preferably between 10° C. and 110° C. to crystallize thediester of the formula I when the partial pressure of the diester in theevaporated gas phase stream is not more than 20 mbar, preferably notmore than 10 mbar, particularly preferred not more than 5 mbar. Theconcentration of the diester of the formula I in the melt stream isadvantageously adjusted to a minimum of 30 wt.%, preferably a minimum of40 wt. %, particularly preferred a minimum of 60 wt. %, in particular aminimum of 70 wt. %. According to a preferred embodiment, the meltstream has a water content of less than 10%, in particular less than 5%,most preferred less than 1%. The method is particularly suitable forpurification of the diester of the formula I being3,6-dimethyl-1,4-dioxane-2,5-dione (dilactide), in particular L,L-dilactide.

According to an advantageous embodiment, at least a part of the diesterof the formula I originates from an upstream purification device, whichcan be in particular stem from at least one of a process stage of thepreparation of polylactide, the polycondensation of lactic acid, thethermal depolymerisation of oligomers of lactic acid with an averagemolecular weight of between 500 g/mol and 5,000 g/mol, the rectificationof dilactide, the ring-opening polymerization of a dilactide-containingreaction mixture, the vacuum demonomerization of polylactide orcopolymers thereof. The upstream purification can involve two or moreprocess stages of the abovementioned processes and/or several of theabovementioned processes simultaneously.

In particular, an alpha-hydroxy-carboxylic acid of the formula I from aalpha-hydroxy-carboxylic acid of the formula II

can be used for the preparation of a biodegradable, intermolecularcyclic diester, wherein R is chosen from hydrogen or one of a linear orbranched aliphatic radical having one to six carbon atoms. According toa particularly preferred embodiment, the alpha-hydroxy-carboxylic acidof the formula II is lactic acid.

The concentration of the alpha-hydroxy-carboxylic acid of the formula IIin the melt stream is advantageously adjusted to a maximum of 20 wt.%,preferably a maximum of 5 wt. %, particularly preferred a maximum of 1wt. %. If the concentration of the alpha-hydroxy-carboxylic acid in themelt stream can be limited to less than 10 wt. % the lactide obtained bythe melt crystallization apparatus can be of a higher purity and thislactide can be fed back into the previous purification step to increasethe purity of the end product, thus the polylactic acid. By this measureit is possible to produce a polylactic acid of a high purity and highmolecular weight.

If the concentration of the alpha-hydroxy-carboxylic acid in thebiodegradable, intermolecular cyclic diester can kept low, it is alsopossible to control the polymerization and to adjust the physical andchemical properties of the biodegradable, intermolecular cyclic diesteraccording to formula I.

In particular a polylactic acid (PLA), particularly a L- or D-polylacticacid (PLLA or PDLA), having a molecular weight of at least 10 000 isobtainable. Advantageously, the molecular weight of the PLA is at least20 000, particularly advantageous a molecular weight of at least 50 000.

The lactide recovered and recycled according to the method of theinvention has a sufficient purity for being re-used in thepolymerization process leading to PLA with the above-mentioned desiredparameters.

A layer crystallization apparatus according to the invention comprises avessel, receiving a melt stream containing a biodegradable,intermolecular cyclic diester of an alpha-hydroxy-carboxylic acidaccording to the formula I,

whereby R is chosen from hydrogen or one of a linear or branchedaliphatic radical having one to six carbon atoms. The layercrystallization apparatus further comprises a heat exchanger having aheat exchange surface a heat transfer medium for cooling the heatexchange surface and a crystallization surface provided on the heatexchange surface for growing crystals of the diester of formula I.

A polymerization plant for the polymerization of the diester accordingto formula I comprises a layer crystallization apparatus according tothe invention. The polymerization plant can further comprise at least apurification apparatus for the biodegradable, intermolecular cyclicdiester according to formula I and/ at least one depolymerizationreactor arranged upstream of the layer crystallization apparatus.

These and other objects and advantages of the invention will become moreapparent from the following detailed description, taken in conjunctionwith the accompanying drawings wherein:

FIG. 1 shows a flow chart of the method according to the invention;

FIG. 2 shows the regeneration of lactide from the evaporated gas phasestream of the devolatization step by means of crystallization;

FIG. 3 shows the regeneration of lactide from the evaporated gas phasestream of the devolatization step by means of desublimation

FIG. 4 shows a phase diagram of lactide

FIG. 5 shows the regeneration of lactide from the evaporated gas phasestream by the raw lactide crystallization step

FIG. 6 shows an embodiment of a crystallization plant

FIG. 7 shows an embodiment of a suspension crystallization plant

FIG. 8 shows an embodiment of a desublimation plant

FIG. 9 shows a first embodiment of a layer crystallization device

FIG. 10 shows a second embodiment of a layer crystallization device

FIG. 1 shows the method for producing PLA from lactide and the ringopening polymerization. The steps in FIG. 1 include a preparation step26 followed by a fermentation step 27 performed in a fermentationapparatus. During the preparation step 26, a biomass feed 80 istransformed into a raw material stream 28. After the preparation step26, the raw material stream 28, containing polysaccharides and/orpolysaccharides is fed into the fermentation apparatus for performingthe fermentation step 27. The fermentation apparatus can be a reactorvessel containing the liquid reaction mixture. If needed, a stirringelement may be foreseen to homogenize the reaction mixture while thefermentation reaction is performed. The fermentation may be performed asa batch process or as a continuous process. The product of thefermentation step leaving the fermentation apparatus is a raw lacticacid in solution 29.

As a next step, the solvent has to be removed from the raw lactic acidin a solvent removal step 30 so to obtain a purified lactic acid 35. Thesolvent can be treated and recycled at least partially to be addedduring the fermentation step 20. The purified lactic acid is subjectedto a pre-polymerization and dimerization step 40 to obtain a raw lactide45.

As a next step the raw lactide 45 is to be purified in a raw lactidepurification step 50. The product of the raw lactide purification stepis a pure lactide 55. The pure lactide stream contains at least 85weight % of lactide. Any lactic acid present in the pure lactide streamis less than 0.2% and any water is present in less than 1%, preferablyless than 0.1%. The pure lactide 55 is processed to raw PLA 65 in a ringopening polymerization step 60. The raw PLA 65 can be further purifiedin a purification step for raw PLA 70 to obtain a pure PLA 75. Anyimpurities are removed from the purification apparatus as a purge 77.

FIG. 2 shows the regeneration of lactide from the evaporated gas phasestream 135 by means of crystallization. FIG. 2 in particular relates tothe treatment of the purge 77 of FIG. 1. In FIG. 2, the steps, whichhave been already discussed in connection with FIG. 1 are not explainedagain. These steps carry the same reference numbers and are notexplained in further detail. The raw PLA 65 from the ring openingpolymerization step 60 is purified in a purification step 170. Thispurification step 170 is performed as a devolatization in a devolatizer.By this purification step a purified PLA 175 is obtained. In adevolatizer, the low boiling fractions from the raw PLA 65 containinglactide are vaporized under vacuum conditions. Thereby the evaporatedgas phase stream 135 is obtained. This evaporated gas phase stream 135is cooled and condensed in a condensation step 140. The condensate 145is fed into a crystallization step 100. During the crystallization stepa pure lactide stream 110 is obtained, which can be fed into the ringopening polymerization step 60 together with pure lactide stream 55. Thepurge 120 from the crystallization step 100 is a waste stream, howeverit is possible to recycle at least a portion thereof to the raw lactidepurification step 50, the pre-polymerization and dimerization step 40 orthe solvent removal step 30.

As an alternative, the devolatization step can be performed in more thanone stage. During each such additional stage an evaporated gas phasestream can be generated. One such additional condensation step 150 isshown in FIG. 2 for an evaporated gas stream 155 from such an additionaldevolatization step. The condensate 156 of this additional condensationstep 150 is fed either into the condensate stream 145 or directly intothe crystallization apparatus for performing the crystallization step100.

FIG. 3 shows a variant of the method as shown in FIG. 2. Thecondensation step 140, 150 and the crystallization step 100 issubstituted by a desublimation step 200. Thus condensation andcrystallization occur in the same apparatus due to the fact that theevaporated gas phase stream is solidified directly from the gas phasestream.

A plurality of desublimation steps may alternatively foreseen, inparticular if a plurality of devolatization steps is foreseen. Anadditional desublimation step 210 is shown in FIG. 3 as optionalalternative in dotted lines. The purge stream 215 is a waste stream,however it is possible to recycle at least a portion thereof to the rawlactide purification step 50, the pre-polymerization and dimerizationstep 40 or the solvent removal step 30.

Such a desublimation is possible in a low pressure region. In the phasediagram for the lactide a phase transition from gas phase to solid phaseis possible along curve 220. The curve 220 extends from the y-axis,which corresponds to a temperature of 60° C. to the triple point 230.When cooling the lactide at a pressure or partial pressure of less than2 mbar, a direct transition from the gas phase to the solid phase takesplace.

FIG. 5 shows a further variant of the method according to FIG. 2. FIG. 5shows the regeneration of lactide from the evaporated gas phase stream135 by means of crystallization. In FIG. 5, the steps which have beenalready discussed in connection with FIG. 1 or FIG. 2 are not explainedagain. The steps, by which the same task as in FIG. 1 or FIG. 2 isperformed, carry the same reference numbers and are not explained infurther detail. The raw PLA 65 from the ring opening polymerization step60 is purified in a purification step 170. This purification step 170 isperformed as a devolatization in a devolatizer. The evaporated gas phasestream 135 containing the low boiling fraction of the devolatizationstep is cooled and condensed in a condensation step 140. The condensate145 is fed into the equipment for performing the lactide purificationstep 50, which can include a crystallization step. A purge streamcontaining the impurities, which should not be present in the PLA can bedrawn off from the lactide purification apparatus to perform the lactidepurification step 50.

The devolatization can be performed in more than one devolatizer. Thecondensation 150 of the evaporated gas phase stream 155 can be performedseparately from the condensation 140 of the first devolatization step.

EXAMPLE 1

A solvent free ring opening polymerization to obtain a raw polylacticacid has been performed in two different tests. The following conditionsapply to the first and second test of example 1: the raw polylactic acidis fed into a purification apparatus for performing a devolatization.The product of the devolatization is a purified polylactic acid and anevaporated gas phase stream containing light boiling compounds such aslactide. The evaporated gas phase stream from the devolatization has alactide content of around 98.5% and is liquefied in a condenser and fedinto a vessel of a layer crystallization apparatus to be solidified toform a solidified mass. The idification takes place by crystallizing thelactide on the heat exchanging surfaces of the layer crystallizationapparatus. Thereafter the solidified mass is molten after having beentransported to the layer crystallization apparatus by heating of thevessel to form a molten mass. Then the molten mass is fed back into theprocess, that is in the ring opening polymerization apparatus.

The crystallization step for this test has been performed twice as shownin table 1a. During the first crystallization step, the molten mass hasbeen crystallised, the liquid residue has been discharged. Then thesolidified mass has been subjected to sweating. The sweating process hasbeen performed in two stages. At the end of each stage, a measurement ofthe point of solidification has been performed. The point ofsolidification of a mixture correlates with the purity of the maincomponent in the mixture according to the phase diagram of lactideaccording to FIG. 4 and consequently allows judging the progress of thepurification. The purity of the lactide reached after the first sweatingstep of the first crystallization step has been 99.5% . A purity of99.6% after the second sweating step of the second crystallization stephas been reached.

For the second test, the analysis for particular impurities, that is Snions and free acids, has been performed for all fractions, that is thefeed, the residue, the sweating fraction and the solidified mass formingthe crystallizate. The results of this second test are shown in table1b. In this test the sweating step has been performed only once.

In a third test, the crystallizate of the second test has been moltenagain and crystallized. In this test, only the residue has beendischarged and a sweating step has not been performed. The results ofthis crystallization are summarized in table 2.

The Sn ions stem from the catalyst. Under free acids, it is intended anyacids which would act as a chain stopper during polymerization.Moreover, the coloring and odors of the feed and the crystallizateobtained by each of the sweating stages are compared to each other intable 1a and table 1b.

TABLE 1a Results of the crystallization of the evaporated gas streamfrom devolatization according to the first test Solidification FractionMass, g point, ° C. Coloring Odor Feed 3560 95.76 yellowish strong,“atypical” Residue 940 92.36 — — Sweating 418 95.95 — — Fraction 1Sweating 314 96.56 — — Fraction 2 Crystallizate 1888 97.08 nearly weak,“typical” colorless

TABLE 1b Results of the crystallization of the evaporated gas streamfrom devolatization according to the second test: Solidification FreeAcid, Fraction Mass, g point, ° C. Sn, ppm mmol/kg Coloring Odor Feed5200 96.07 13 72 yellowish strong, “atypical” Residue 808 90.50 52 274 —— Sweating Fraction 875 96.01 14 71.9 — — Crystallizate 3517 97.01 322.2 nearly weak, colorless “typical”

TABLE 2 Results of the repeated crystallization: Solid- Free Mass,ification Acid, Fraction g Point, ° C. Sn, ppm mmol/kg Coloring OdorFeed 3240 97.01 3 22.2 nearly weak, colorless “typical” Residue 136796.47 6 54.4 — — Crystallizate 1873 97.15 <2 7 colorless weak, “typical”

EXAMPLE 2

Desublimation

In this test the separation effect of the desublimation the purity ofthe lactide has been checked.

The evaporated gas phase stream from the ring opening polymerization,which was also used for the tests of example 1, has been fed into a tubehaving an inner diameter of 50 mm and a length of 3 m, in which thelactide has been desublimised, thus solidified directly from the gasphase to form a crystallizate on the heat exchanging surfaces of thelayer crystallization apparatus, which has been employed for the testsaccording to example 1. The residue has been fed back into the mainprocess stream thus a subsequent devolatization stage.

A solid layer of a thickness between 10 and 15 mm has been produced anddeposited on the inner surface of the tube. When the desublimation hasbeen finished, a portion of the deposited solid layer has beendischarged from the tube and molten to form a molten mass. Thesolidification point of this molten mass has been determined. Thesolidification point has been measured and was 96.97° C. The solidifiedmolten mass was nearly colourless and had only a weak odour.

Each of the test results of the first and second examples show that thepurification of the lactide of the evaporated gas stream to besufficiently close to the melting point of pure lactide. The purity ofthe lactide obtained by desublimation according to this example wasabout 99.5%. For L-lactide the melting point is at 97.7° C.

The tests have been conducted in a lab test static crystallizer with thebelow-mentioned design details. A static crystallizer is a specialembodiment of a layer crystallization apparatus in which the melt is notsubjected to any forced convection during the crystallization. The teststatic crystallizer consists of a vertically arranged jacketed 80 mmdiameter tube with a length of 1,200 mm and having a rated volume of 6I. The tube has a tightly closing lid at the top allowing filling theinput melt into the tube and to close the tube tightly during thecrystallization. At the bottom, the tube diameter is reduced to 20 mmand there is an outlet valve placed directly below the passage ofreduced diameter. The valve allows the liquid fractions be drained outof the tube by gravity. In the jacket of the tube, a heat transfermedium is circulated that supplies the cooling or heating energies forthe crystallization and subsequent sweating and melting steps. The heattransfer medium is either heated or cooled in a commercial thermostatapparatus with time-programmable temperature profiles.

After filling the input melt mass into the crystallizer tube, thefilling aperture is closed. The heat transfer medium temperature is thendropped to a value for start of crystallization and then it is decreasedaccording to programmed temperature/time profile to the final value ofcrystallization. During this cooling, the crystals nucleate and startgrowing upon the inside wall of the crystallization tube. Aftertermination of the crystallization, the non-crystallized residue isdrained to a receiver container by opening the drain valve at the bottomof the tube. The sweating fraction is collected to different containers,if required in several cuts. After the sweating has been finished, thedrain valve is closed, and the remaining crystals are molten and drainedout of the crystallizer tube to the corresponding container by againopening the drain valve.

When operated the two first stages have been subjected to the followingoperating conditions: The crystallizer tube has been pre-cooled to 95°C. for the start of the crystallization. The temperature of the heattransfer medium has been gradually decreased to the final value of 90°C. within six hours thereafter. During this period the crystallizationof the lactide on the heat exchange surfaces has been performed. Themelt has been kept in the vessel of the crystallization apparatus toallow for the growth of the crystals. When the crystallization has beencompleted, the drain valve has been opened to discharge the liquidresidue, thus the mother liquor.

After opening the drain valve for the residue drainage, the temperatureof the heat transfer medium has been gradually increased to 98° C. toperform a sweating step. The sweating step has lasted for five hours.After having completed the sweating step the liquid residue has againbeen discharged by opening the drain valve.

Subsequently the crystallizate has to be removed from the heatexchanging surfaces of the layer crystallization apparatus. The meltinghas been performed at a temperature of 120° C. During the melting thedrain valve is held closed and opened only after completion of themelting step for discharging the melt from the crystallization vessel.

During the second stage, the crystallizer tube was pre-cooled to 96° C.for the start of the crystallization. The temperature of the heattransfer medium was then gradually decreased to the final value of 92°C. within six hours. After opening the drain valve for the residuedrainage, the temperature of the heat transfer medium was graduallyincreased to 98° C. at the end of sweating. The sweating lasted fivehours. The melting performed at a temperature of 120° C.

Solvent-free melt crystallization is used in a commercial scale. Acrystallization apparatus comprising falling film crystallizers asdescribed e.g. in U.S. Pat. No. 3,621,664 is commercialized by SulzerChemtech Ltd. Switzerland.

Alternatively the crystallization apparatus can comprise staticcrystallizers as described in e.g. EP0728508 (A1); EP1092459 (B1);EP0891798 (B1) and is commercialized by e.g. Litwin, France; SulzerChemtech Ltd., Switzerland. The static crystallizer essentially consistsof a tank, in which the crystallized melt is filled in and of coolingsurfaces being cooled/heated from the inside by a heat transfer medium.The heat transfer medium circulate in a vertical plate bundle as shownin FIG. 9 or a tube bundle as shown in FIG. 10. The crystals grow on theexternal walls of these heat exchanging surfaces.

Alternatively the crystallization apparatus can comprise a suspensioncrystallization apparatus as described e.g. in U.S. Pat. No. 6,719,954B2; EP 1 245 951 A1; U.S. Pat. No. 6,241,954 B1; U.S. Pat. No. 6,467,305B1; U.S. Pat. No. 7,179,435 B2; US 2010099893 (A1) and is commercializedby GEA Messo PT, Germany and Sulzer Chemtech Ltd. Switzerland. In such asuspension crystallization apparatus small crystals are created, whichgrow in suspension in a growth vessel. The growth vessel and thesuspension crystallization apparatus may be merged together as one unit.The slurry is then conveyed to a wash column where the crystals arewashed by counter currently flowing, partly returned molten crystalfraction and the wash liquid, being loaded with the non-desiredcomponents is rejected as residue. The residue of as first suspensioncrystallization apparatus may be collected and recrystallized and washedagain in a second suspension crystallization apparatus of similarconfiguration so as to recover any lactide from the residue of the firstassembly.

In FIG. 6, a melt layer crystallization apparatus comprising a staticplate bundle crystallizer 1 is shown. The configuration of such acrystallizer can have the same or corresponding elements to thecrystallization apparatus as shown in FIG. 9. The crystallizer 1 isloaded with a batch of molten mass to be crystallized by a line 2 bymeans of a pump 3 from the lactide feed vessel 4. The feed is coming tothe feed vessel by a feed line 5. This feed can be either a gaseousstream or a melt stream. In particular, the feed may be an evaporatedgas phase stream from a devolatization unit (70, 170) as shown in FIG.1, 2, 3, 5.

The residue of the crystallizer 1 as well as the sweat fraction and themolten crystal fraction are drained to the appropriate vessels 6, 4 and7, respectively, by outlet line 8 and drain valve 9. A header 10 withnecessary valves allows to direct the particular fractions being drainedto the appropriate vessels. The header has the function of a liquiddistributor. The residue is collected in vessel 6. The molten crystalfraction, which contains the purified lactide is drained to vessel 7.The residue and purified lactide can be transferred to theirdestinations by the transfer pumps 11 and 12. The sweat fraction can becollected in vessel 6 and discharged in the same way as the residue orit can be collected in vessel 4 for being recycled to the crystallizer 1by line 2. The plate bundle as shown in FIG. 9 is cooled and heated byheat transfer medium coming by line 21 and leaving the bundle by line22. The circulation pump 23 allows the heat transfer medium becontinuously circulated in the energy system. The cooling and heatingenergies are supplied via both heat exchangers 24 and 25. The here shownheat exchangers represent only one, simple possibility of the energysupply to the crystallization system. There are other solutionspossible, like systems with energy buffer vessels and other energysupply systems being well known to a person skilled in the art from theindustrial practice.

In the embodiment according to FIG. 7, the liquefied lactide from thedevolatization is fed continuously via line 301 into the crystallizationsection of the melt suspension crystallization apparatus. The meltsuspension crystallization apparatus comprises a crystallizer and/orscraper unit 302 and a vessel 303 for growing crystals. A transfer line305 leads from the crystallizer 302 to the vessel 303, a transfer line306 from the vessel 303 to the crystallizer 302. A circulation pump 304may be arranged in the transfer line 306, which allows the slurry to becirculated between the crystallizer 302 and the vessel 303. Thecrystallizer and/or scraper unit has a cooling jacket 321 for coolingthe crystallizer unit walls. Crystal nuclei on the internal wall areformed on the inner wall surfaces of the crystallizer 302. The crystalnuclei are then scraped continuously from the internal wall surfaces bya scraper element 322. The crystal nuclei are allowed to grow whilebeing suspended in the melt, which is a lactide melt in accordance withthe preferred application.

In an alternative version, both devices, the crystallizer 302 and thevessel 303 may be combined into one common unit. The lactide feed mayalso be directed to the crystallizer 302, or to one of the circulationline 305 or transfer line 306 instead of the vessel 303. The designdetails of commercially available melt suspension crystallizationdevices are known to a person skilled in the art.

A part stream of the slurry is split from the circulation line 306 toline 307 feeding to the wash column 308. The flow rate of this partstream is controlled by a valve 309. The flow rate is essentially thesame as the flow rate of the feed of line 301. In the wash column 308,the crystals contained in the slurry are forced to move towards one headof the wash column and the residual melt moves towards the opposite end.The crystals are moved by a mechanical element 310 like screw conveyoror by a piston with a sieve-shaped head, which repeatedly forces thecrystals in one direction allowing the melt to pass in the oppositedirection. In another type of commercially available wash column 308,the required crystal and melt flow patterns are established byappropriate design of vessel internals in such a way that no moveableparts are needed.

The crystal slurry is directed by the mechanical element 310 to a columnend, in this example the bottom end or sump and then discharged to thecirculation loop 311. A forced circulation of the crystal slurry ismaintained by the circulation pump 312. The crystal slurry flows thenthrough the melter 313, in which the crystals are molten to from amolten mass. One part of that molten mass is continuously discharged viathe discharge line 314 and the control valve 315. This part is in thepreferred application in a polymerization plant for the production ofpolylactic acid the purified lactide that is then returned to thepolymerization reactor or the devolatization. The remaining part flowsvia return line 316 back to the wash column. This part is used formaintaining the countercurrent flow of crystals and melt within the washcolumn. At the other end of the wash column, here the column head, theresidual melt is taken out of the column via line 317 and valve 318.This residual melt is the purge stream.

In the embodiment according to FIG. 8, the lactide vapor comes via thesupply line 401 from the devolatization stage via opened valve 402 andthe branch line 403 to the solidification device 404 where is solidifiesupon the cooled surfaces 405. The solidification device can be forexample at least one of a desublimation unit or a crystallizer. Thenon-solidified residual vapor can flow via line 406 back to the mainprocess stream, e.g. to the second devolatization stage or be discarded.The heat exchange system is similar to the one as disclosed in FIG. 6and is not further described here. Reference is made to the descriptionof FIG. 6.

After a portion of the gaseous stream has solidified on in the heatexchanging surfaces of the solidification device 404, the valve 402closes and the valve 407 opens to direct the vapor to the secondsolidification device 408 in which the solidification of the vapor isperformed. The second solidification device essentially works in thesame manner as the solidification device 404.

The solidification device 404 is pressurized by allowing an inert gas,e.g. nitrogen to flow in via valve 409 to increase the working pressurefor melting the solidified mass. This solidified mass contains accordingto the preferred application for the purification of lactides thelactide fraction and is the crystallizate. The heat exchange surfacesare now heated by a heat transfer medium to melt the solidified mass toform a molten mass. The molten mass, in particular the molten lactide isdumped via valve 410 to the collecting vessel 411 from where it can beconveyed by pump 412 to the polymerization or devolatization stages.

After having molten the solidified mass, the drain valve 410 closes andthe solidification device 404 is evacuated by valve 413 and line 406before starting the subsequent solidification.

There are minimum two solidification devices necessary to assurecontinuous lactide vapor reception, however the number of such devicescan be higher and is not limited.

If no subsequent devolatization stage is foreseen, the residue is awaste stream which consequently is to be treated in a waste treatmentprocess. Optionally a sweating step may be foreseen. The heat exchangesurface may be advantageously formed as a tube, which is disposed with acooling mantle. If the solidification device is configured as a fallingfilm crystallization apparatus, it may be configured as shown in FIG.10. By means of the cooling mantle, the temperature generated on theinner surface of the tube is kept below the sublimation point for thegiven partial pressure of the vapour to be desublimised, in particularthe lactide.

FIG. 9 shows an embodiment of a layer crystallization apparatus. Thecrystallization apparatus 250 has a container 253 for the reception ofthe melt which contains the lactide and the impurities to be removedfrom the lactide, that is the product of the devolatization namely anevaporated gas phase stream or a melt stream thereof. A plurality ofwall elements 255 are arranged in this container 253 whereby the wallelements are spaced apart from one another. The wall elements 255contain closed channels 257 for the circulation of a fluid heat exchangemedium. These wall elements are also called plate bundles. Each wallelement 255 is selectively heatable or coolable by circulation of thetemperature fluid heat exchange medium in the interior of the closedchannels 257. The closed channels 257 open into an inlet tank 259 and anoutlet tank 260, which serve for the distribution of the fluid heatexchange medium to the Individual channels 257 or for the reception offluid heat exchange medium from the individual channels.

The intermediate spaces 256 between the wall elements 255 are filled inoperation with the melt which contains the lactide to be purified. Themelt is distributed over the totality of the wall elements via inflows261 which open into inflow distribution elements 262 so that the wallelements 255 are surrounded all over by melt. After the filling of thecrystallization apparatus 250 with melt, fluid heat exchange medium isconducted as coolant through the channels 257, whereby the wall elements255 are cooled. The melt crystallizes at the wall elements 255 to acrystallization layer whose thickness increases continuously. Due to thedifferent melting points of the individual lactide and the impurities inthe melt, the crystallizate layer contains a higher portion ofhigh-melting lactide. The solid lactide is deposited from the start atthe crystallization surfaces of the wall elements 255, which means thatit is therefore concentrated in the crystallizate layer. If the melt iscooled further, impurities with somewhat lower melting points may alsostart to crystallize.

A larger portion of the impurities remains in the liquid phase and islet out via outflows which are located in the base region 264 of thecrystallization device 250. The liquid phase is also called the motherliquor. The impurities melting at a lower temperature than the lactideare concentrated in the mother liquor. The mother liquor in this casecontains a waste product.

The wall elements 255 are heated again in the second phase of thecrystallization. During this second phase, also a partial melting of thecrystallizate layer, the so-called sweat phase, may take place. Afraction lactide containing still some impurities resulting frominclusions of mother liquor between the crystal surfaces during crystalgrowth can be selectively separated during the sweat phase. Thecrystallizate layer substantially remains connected to the wall elementsin the sweat phase; only individual melt drops are drawn off. Thelow-melting impurities, which have just been freed by the partialmelting of the crystals, art concentrated in these first drops. A veryselective separation of impurities is thus possible in the sweat phase.The temperature on the surface of the wall elements 255 preferablyincreases continuously during the sweat phase. In this case, a pluralityof fractions can also be drawn off during the sweat phase.

In the third phase, the melting off of the crystallizate layer takesplace, that is the removal of the crystallizate from the wall elements255. For this purpose, the channels 257 in the wall elements 255 arecontacted with fluid heat exchange medium which is used as a fluidheating medium.

FIG. 10 shows a falling film crystallization apparatus 270. The fallingfilm crystallization apparatus 270 comprises a container 271 containingplurality of tubes forming a tube bundle 272 . The container receivesthe lactide from the devolatization which is fed into the container asan evaporated gas phase stream or a melt stream. The feed stream entersthe crystallization apparatus via inlet tube 273. The tubes of the tubebundle 272 are hollow so as to form a passage for a heat exchange fluid.The heat exchange fluid enters the tube bundle via an inlet conduit 275and leaves the tube bundle via an outlet conduit 276. The inlet conduitopens into a fluid distribution element being in fluid connection withthe passages of the tubes of the tube bundle. The passages of the tubesare received in a fluid collection element being in fluid connectionwith the outlet conduit 276.

The heat exchange fluid can be a heating fluid or a cooling fluid,depending on the mode of operation of the crystallization apparatus. Inthe crystallization mode, a cooling fluid is circulated in the tubes,thus lowering the temperature of the outer surfaces of the tube withrespect to the feed temperature. The temperature is lowered so as tocrystallize the compounds having the highest melting points. The liquidfraction, which is not crystallized , thus the mother liquor, leaves thecontainer in the sump, when the crystallization apparatus is incrystallization mode. Under crystallization mode it is intended theperforming of the crystallization step. Both of the crystallizationapparatuses of FIG. 9 and FIG. 10 are designed for a batch operation.That means that after the crystallization step is performed, a meltingstep is performed to melt the crystal fraction and drain it to the sumpfrom which it is discharged by the discharge tube 274. The crystalfraction is deposited during the crystallization mode onto the externalsurfaces of the tubes of the tube bundle.

The falling film crystallization allows for a faster crystallizationthan the melt crystallization apparatus using wall elements in the formof plate bundles.

1. A method to prepare a polylactic acid comprising the steps of: (i)performing a ring opening polymerization using a catalyst and either acatalyst killer compound or an endcapping additive to obtain a rawpolylactic acid of MW greater than 10,000 g/mol, (ii) purifying the rawpolylactic acid by removing and separating low boiling compoundscomprising lactide and impurities from the raw polylactic acid bydevolatization of the low boiling compounds as a gas phase stream, (iii)purifying the lactide from the devolatization and removing theimpurities from the gas phase stream of evaporated low boiling compoundsby means of condensing the evaporated gas phase stream to give acondensed stream and a subsequent melt crystallization of the condensedstream, wherein the lactide is purified and the removed impuritiesinclude a catalyst residue and a compound containing at least onehydroxyl group such that the purified lactide is polymerized by feedingit back into the ring opening polymerization.
 2. The method of claim 1,wherein the melt crystallization is performed by means of a layer or asuspension crystallization.
 3. The method of claim 1, wherein theevaporated gas phase stream from the devolatization contains at least30% of lactide, preferably at least 60%, most preferred at least 90%. 4.The method of claim 1, wherein a crystal resulting from the meltcrystallization of the condensed stream is crystallized in a furthercrystallization stage.
 5. The method of claim 1, wherein the layercrystallization comprises a sweating step followed by a melting step ofa solidified fraction present in a crystalline form on a crystallizationsurface.
 6. The method of claim 1, wherein the removed impuritiesinclude either an organometallic compound or a carboxylic acid.
 7. Themethod of claim 1, wherein an apparatus is used for the meltcrystallization which does not have an inert gas stream.
 8. The methodof claim 1, wherein at least a portion of a purge stream from thecrystallization is recycled to a raw lactide purification step, apre-polymerization and dimerization step, or a solvent removal step inthe production of a purified lactide.
 9. The method of claim 1, whereina mother liquor from the crystallization and/or a liquid from thesweating step is collected and recrystallized in order to recover thelactide.
 10. An apparatus for carrying out the method of claim 1,comprising: a polymerization reactor for performing a ring openingpolymerization to obtain a raw polylactic acid, a devolatizationapparatus for removing and separating low boiling compounds comprisinglactide and impurities from a raw polylactic acid, and a crystallizationapparatus for purifying a lactide and removing impurities from acondensed stream, wherein a condenser for condensing a gas phase streamto give a condensed stream is arranged between the devolatizationapparatus and the crystallization apparatus.
 11. The apparatus of claim10, wherein the crystallization apparatus is a layer crystallizationapparatus or a suspension crystallization apparatus.
 12. The apparatusof claim 11, wherein the layer crystallization apparatus is a static ora falling film crystallization apparatus.
 13. The apparatus of claim 11,wherein the suspension crystallization apparatus contains a wash column.